The invention relates to a ceramic capacitor comprising at least two electrodes and a ceramic dielectric of a dielectric, ceramic preparation, which is essentially composed of an oxide-ceramic dielectric substance and a sintering aid.
In ceramic capacitors, a ceramic material, i.e. an inorganic polycrystalline solid produced by firing at high temperatures, is used as the dielectric. Ceramic capacitors are available as disc capacitors, tubular capacitors and multilayer capacitors. With a view to miniaturization, in particular ceramic multilayer capacitors are important. Said capacitors are composed of a monolithic ceramic block into which electrodes are sintered in accordance with a comb-like pattern.
Ceramic multilayer capacitors are customarily manufactured by mixing, in a first step, the pulverized ceramic starting material for the ceramic dielectric with a water-containing binder preparation so as to form a pulpy suspension. This suspension is scraped on a belt conveyor so as to form a thin layer. As a result of evaporation of the solvent, a ceramic foil is formed which can be cut into sheets onto which electrodes can be printed by means of screen printing. After printing electrodes on said sheets, the sheets are disposed in layers so as to form stacks of 30 to 60 sheets. These stacks are cut into small cuboids, which are fired at temperatures in the range from 1000xc2x0 C. to 1400xc2x0 C. The contacts are manufactured, for example, by immersing in a metal-ceramic paste followed by a firing process.
The quality of multilayer capacitors is determined by the chemical composition of the material used for the ceramic dielectric and/or for the electrodes as well as by the process conditions. As regards the process conditions, particularly the sintering conditions are very important.
A plurality of inorganic binary and multiple oxides in different preparations are known as the ceramic dielectric materials for ceramic capacitors. For example, class 1 ceramic capacitors customarily contain a mixture of binary oxides of lanthanum and titanium, class 2 ceramic capacitors customarily contain a mixture of ferroelectric, titanium and zirconium-containing perovskites, and class 3 ceramic capacitors customarily contain polycrystalline barium titanates and strontium titanates.
In the sintering operation, various opposed oxidation and reduction reactions may take place in these binary or multiple oxides dependent upon the sintering atmosphere. If the sintering operation takes place in a reducing atmosphere, particularly titanium oxide and titanates may become semiconducting. In this semiconducting state, they cannot be used as a dielectric. However, sintering under oxidizing conditions can take place only if the electrode material consists of a non-oxidizable noble metal having a high melting point, such as rhodium, palladium or platinum. However, rhodium, palladium and platinum are very expensive; up to 50% of the cost of manufacturing a multilayer capacitor may be attributable to these noble metals. Therefore, there is a tendency towards replacing rhodium and platinum as the electrode metal by silver, copper or their alloys, which are much cheaper metals. Since the melting point of silver and copper is much lower, a sintering temperature is required that is much lower than the customary sintering temperature.
It is well known that the sintering temperature can be reduced by accelerating the extremely slow diffusion transport in the solid oxide phases. A substantial acceleration of the diffusion transport is achieved by the presence of liquid phases in the sintering process. Liquid-phase sintering requires sintering aids as additives, which form liquid phases whose melting point is as low as possible in order to obtain an effective reduction of the sintering temperature.
When use is made of sintering aids, it is also very important that the electrical properties of the ceramic dielectric material are not adversely affected by the additives. Sintering aids that are known to be used for liquid-phase sintering of perovskite phases are glass-forming mixtures of CdOxe2x80x94ZnOxe2x80x94Bi2O3xe2x80x94PbOxe2x80x94B2O3xe2x80x94SiO2, as disclosed in U.S. Pat. No. 3,811,973
A drawback of boron oxide-containing sintering aids is, however, that during mixing the suspension, they react with the water-containing binding agent. The boron oxide hydrolyzes with the water from the binder preparation to boric acid B(OH)3. The boric acid thus formed can react with the organic constituents of the binder preparation, thereby causing polymerizations in the binder. Polymerization of the binder causes the suspension to be destabilized. As a result, shaping of the green material is hampered and the electrical properties of the fired and sintered dielectric material are changed.
Therefore, it is an object of the invention to provide a ceramic capacitor comprising at least two electrodes and a ceramic dielectric of a dielectric, ceramic preparation, which is essentially composed of an oxide-ceramic dielectric substance and a sintering aid, which ceramic capacitor can be sintered at low temperatures and is characterized by reproducible dielectric properties.
In accordance with the invention, this object is achieved by a ceramic capacitor comprising at least two electrodes and a ceramic dielectric of a dielectric, ceramic preparation which is essentially composed of an oxide-ceramic dielectric substance and a sintering aid including zinc borate Zn4B6O13. Such a capacitor can be sintered at low temperatures and hence can be provided, in particular as a multilayer capacitor, with base metal electrodes. A small quantity of the sintering aid is sufficient, so that the dielectric properties of the capacitor are hardly influenced. It is particularly advantageous that the zinc borate in the sintering aid does not react with water and the binder to undesirable by-products.
In accordance with a preferred embodiment of the invention, the oxide-ceramic dielectric material is a manganese-containing calcium-strontium-titanium zirconate of the general formula (Ca1-xSrx)a[Zr1-y-zTiyMnz]O3, where 0.985xe2x89xa6axe2x89xa61.015, 0 less than xxe2x89xa60.08, 0 less than yxe2x89xa60.05 and 0 less than zxe2x89xa60.02. Apart from the very high temperature stability of ∈r, such a capacitor is also characterized by low losses, so that it can suitably be used for applications wherein time-critical quantities are defined by means of capacitors such as time-function elements, oscillation circuits and filters.
It is particularly preferred that the oxide-ceramic dielectric material is a manganese-containing calcium-strontium-titanium zirconate of the general formula (Ca0.937Sr0.063)a[Zr0.938Ti0.040Mn0.022]O3.
In accordance with another embodiment of the invention, the sintering aid may additionally comprise a compound selected from the group formed by CaO, CuO, SiO2, CaSiO3, ZnO and ZnSiTiO5.
The invention also relates to a method of manufacturing a ceramic capacitor comprising at least two electrodes and a ceramic dielectric of a dielectric, ceramic preparation which is essentially composed of an oxide-ceramic dielectric material and a sintering aid including zinc borate Zn4B6O13, by co-sintering said dielectric, ceramic preparation and the electrodes at a temperature in the range from 970xc2x0 C. to 1050xc2x0 C.
The invention further relates to a dielectric, ceramic preparation which is essentially composed of an oxide-ceramic dielectric material and a sintering aid comprising zinc borate Zn4B6O13.
The dielectric ceramic preparation may be used for LTCC substrates (xe2x80x9cLow Temperature Co-fired Ceramicxe2x80x9d) in integrated micromodules.
These and other aspects of the invention will be apparent from and elucidated by means of a drawing and three exemplary embodiments.